Attack Chain

1. A client with a blocked IP address attempts to access a protected resource.
2. The client’s traffic is routed through a trusted proxy, CDN, or load balancer.
3. The trusted proxy forwards the request to the Caddy web server.
4. Caddy Defender receives the request and evaluates the IP address for blocking.
5. Defender incorrectly uses r.RemoteAddr, which reflects the trusted proxy’s IP address, not the client’s.
6. Since the proxy’s IP is not blocked, Defender allows the request to proceed.
7. The client successfully accesses the protected resource, bypassing the intended IP-based restriction.
8. The attacker gains unauthorized access to sensitive information or performs actions they should be restricted from.

Impact

Successful exploitation of this vulnerability (CVE-2026-46415) enables unauthorized access to protected resources by clients that should be blocked based on their IP address. This bypass can lead to data breaches, service disruption, or other malicious activities, depending on the resources protected by Caddy Defender. The severity is high because it directly undermines the intended security functionality of Caddy Defender when deployed behind trusted proxies.

Recommendation

• Upgrade Caddy Defender to version v0.10.1 or later to remediate the CVE-2026-46415 vulnerability, as mentioned in the advisory.
• Deploy the Sigma rule “Detect Caddy Defender IP Bypass Attempt” to identify potential exploitation attempts by monitoring for requests originating from known blocked IP ranges based on web server logs.
• Until upgrading, enforce equivalent IP blocking at the trusted proxy, CDN, load balancer, or firewall layer as a workaround, as suggested in the advisory.